Various types of electronic memory have been developed for computers and similar systems, and each type had specific advantages. For example, dynamic random access memory (DRAM) has a high storage density such that a great amount of data can be stored in a small area. Static random access memory (SRAM) has lower storage density than DRAM but can quickly store and retrieve data.
Other memories include electrically erasable programmable read only memory (EEPROM) and electrically programmable read only memory (EPROM). EEPROM can be easily erased without extra exterior equipment but with reduced data storage density, lower speed, and higher cost. EPROM, in contrast, is less expensive and has greater density but lacks the erase characteristics of EEPROM.
A newer type of memory called “Flash” EEPROM, or Flash memory, has become extremely popular because it combines the advantages of the high density and low cost of EPROM with the ability to electrically erase of EEPROM. Flash memory can be rewritten and can hold its contents without power. Flash memory is a non-volatile memory, which means it can store data and not lose the data when power is lost. It is used in many portable electronic products, such as cell phone, portable computers, voice recorders, etc. as well as in many larger electronic systems, such as cars, planes, industrial control systems, etc.
Conventionally, Flash memory is constructed of many Flash memory cells where a single bit is stored in each memory cell and the cells are programmed by hot electron injection and erased by Fowler-Nordheim tunneling. However, increased market demand has driven the development of Flash memory cells to increase both the speed and the density. Newer Flash memory cells have been developed that allow more than a single bit to be stored in each cell.
One memory cell structure involves the storage of charge in nitride traps surrounded by oxide to form an oxide-nitride-oxide (ONO) stack. This structure is referred to as Silicon-Oxide-Nitride-Oxide-Silicon (SONOS).
Each type of memory has specific applications in which it is optimal. However, each requires a different manufacturing process and specialized processes are required to achieve optimal performance in the optimal applications.
In addition, it would be desirable to optimize the performance of each type of memory on the single semiconductor chip.
However, all these different types of memory and their associated circuitry still have major problems. While the general population believes that electronic devices with the different types of memory work until they become obsolete, this is actually not the case. Most memories can degrade over time and fail over relatively short periods of time. Compared to hardcopy materials that last almost indefinitely, a memory system will degrade over time depending upon the number of program and erase cycles the memory is subjected to over its life.
These problems with degradation and failure have become worse as the electronics industry seeks to make smaller and smaller memory to operate at even less power and even greater speeds.
One source of the problems relates to providing the proper stress to the memory during the erase cycle. Providing a voltage during a program-erase cycle for a period of time stresses the memory cells. This stress can reduce the endurance of the memory cell as program-erase cycles increase over the life of the memory.
Solutions to these problems have been long sought and some solutions are accepted by the industry, such as multi pulse program/erase scheme. But such solutions always require very complex circuit designs.